Mary Hillyer Seaverns and Robert Jean Saner II plan to marry on October 7. Mr. and Mrs. Appleton Hillyer Seaverns of Suffield, Conn. have announced the engagement of their daughter to the son of Mr. and Mrs. Robert J. Saner of Springfield, Ill. The wedding will be held in the Colonial Room of the Horace Bushnell Memorial Hall in Hartford - of which Mr. Seaverns, headmaster emeritus of Suffield Academy, is president.

Miss Seaverns, presented in 1968 at the Holly Ball in Hartford, is an alumna of the Loomis-Chaffee School; graduated in 1973 from Connecticut College, and is the 1977 recipient of a master's degree from Tulane University. She is currently employed by George Washington University. Her grandparents are the late Mr. and Mrs. Charles Frederick Taff Seaverns of Hartford, and the late Mr. and Mrs. Benjamin Hodges of Terrell, Tex.

Mr. Saner, a 1965 valedictorian at the Lawrenceville School, graduated cum laude in 1969 from Princeton University, and in 1972 from Harvard Law School. He is a partner in the Washington, D.C. law firm of White, Fine, and Verville. His father is the retired president of the Springfield (Ill.) Marine Bank, and his grandparents are the late Mr. and Mrs. Harry Edward Saner of Springfield, and the late Mr. and Mrs. George Bent Burroughs of Edwardsville, Ill.